Field of the Invention
This invention relates to an optical communications system comprising an end terminal unit, a plurality of optical communications devices such as an optical amplifier units, and an optical transmission media such as optical fibers connected to the optical communications devices used in an optical communications system and long-distance optical transmission system.
Along with demands for optical communications systems of low cost, optical transmission frequency multiplexing systems are being studied for transmission of two or more multiplexed optical signals of different wavelengths on one optical fiber. The optical amplifier is well suited for use as an amplifier in multiplexed optical transmissions on account of the capability to amplify signal with low noise and having a wide available bandwidth for amplification.
In the rare-earth doped optical fibers and semiconductor amplifier comprising the optical amplifier, the gain is dependent on wavelength so that after amplification, a deviation appears in the optical output or gain between each wavelength. The deviation between wavelengths in particular, is summed in the multiple stages of the optical amplifier, so that a large deviation in optical power occurs after transmission. As a result, after transmission, the wavelength signal of the lowest power from among the multiplexed wavelengths must be considered as the receive power lower limit value. In other words, the maximum distance of a relayed transmission is limited by the wavelength signal having the lowest power.
Accordingly, when inputting multiplex signal into end terminal units or optical relay amplifier units, it is important that an optical transmission system be provided having no gain deviation due to the wavelength so that the maximum relay transmission distance can be expanded.
Technology relating to the above is known as the xe2x80x9cSociety of Electronic Information Communication Signal Transmission Techniquexe2x80x9d OCS94-72, OPE94-95 (1994-11) in xe2x80x9c10 Gbit/s, 4 ch. 200 km, 16 ch. 150 km, 1.3 um zero dispersion fiber relay transmission testxe2x80x9d in the method shown in FIG. 1. In the figure, the reference numeral 82 denotes a distributed feedback laser diode (DFB-LD) used as the light source. The polarization of the light from each DFB-LD is fixed by a polarization controller 83.
The deviation in frequency gain on the receive side can be compensated by setting the optical power of this DFB-LD. In other words, in order to simplify pre-emphasis, the four light sources on both sides (ch.1 through ch.4, and ch.13 through ch.16) are merged by means of a 4xc3x971 coupler 84, and the eight light sources in the center (ch.5 through ch.12) are merged by using an 8xc3x971 coupler 85 having greater loss. The signals from these 16 diodes merged using a 3xc3x971 coupler 86 and then strongly modulated for 10 Gbit/s NRZ (223xe2x88x921) by means of an LN(LiNbO3) modulator 87. The optical signal is amplified to +21 dBM (total optical output) by a high output optical post amplifier 88 utilizing four 1.48 um laser diodes and the result input to a single mode fiber 89.
After amplification with a 0.98 um common optical pre-amp 90 on the receive side, batch dispersion compensation of the 16 signals is performed by means of a dispersion compensation fiber (DCF) 91. After splitting the signals with a 1xc3x9716 splitter 92, the output is passed through a 0.8 nm and 0.3 nm interference filter 93 of three decibels in width to eliminate ASE noise and frequency selection then performed. The optical amplifier 95 is inserted between two types of optical filter for the purpose of compensating the gain tilt in the optical amplifier 90 and maintaining the input power to the O/E converter 94 at a constant level.
FIG. 2 shows an optical spectrum of the 16 WDM signal obtained with the system configuration of FIG. 1. FIG. 2(a) is the optical spectrum prior to input to the postamplifier 88, on which the approximately 10 dB difference in maximum levels is due to the application of pre-emphasis. In FIG. 2(b), the optical spectrum after passing the signal through DCF91 is shown. A 13 dB difference in level occurs due to the gain tilt in the optical amplifier 90. However, due to the effect of pre-emphasis, the ratio of signal to ASE noise is nearly the same value for each channel.
As related above, the optical loss at each wavelength during transmission varies due to the difference in fiber loss over the relay space and the difference in optical power between adjacent wavelengths, etc. In fact, the fiber loss over the relay space and the space within the fiber is not always a fixed amount during actual use so that the estimating the total optical signal power after transmission, and the deviation between wavelengths and optical power of each wavelength is difficult. The total optical signal power, and the deviation between wavelengths and optical power of each wavelength fluctuate due to temperature variations and deterioration over time. Further, non-uniformities in the equipment comprising the different optical systems will cause differences in test equipment reading to occur when measuring total optical signal power, deviation between wavelengths and optical power of each wavelength and the system transmission functions may easily be lost due to changes in transmission conditions of the optical transmission system.
In order to achieve a practical optical transmission system, an optical transmission system is required in which the optical output or the gain, or both are easily controllable and not dependent on total signal optical power, or deviation between wavelengths and optical power of each wavelength after transmission.
In view of the above problems, it is therefore an object of this invention to provide an optical transmission system of high reliability that is both practical and operates stably as a frequency multiplexing optical transmission system by providing an automatically controlled optical communications device whose total pre-established optical signal power, deviation between wavelengths and optical power at each wavelength can be controlled as needed after transmission.
In order to resolve the above mentioned problems, one basic feature of this invention in an optical transmission system configured to connect optical communications devices by means of an optical transmission medium, wherein the optical transmission system is comprised of: a device to detect the status of the optical signal within the optical transmission system, a device to allot the optical control information equivalent to the detected status, a detection device to detect the allotted optical control information, and a device to control the optical signal according to the detected optical control information.
Also, in order to resolve the above mentioned problems, this invention is also characterized by an optical communications device such as an optical amplifier relay unit, that is, an optical line amplifier unit, or an end terminal unit connected to an optical transmission medium, wherein the optical transmission system is comprised of: an optical power adjustment device for adjusting the optical power, a transmission medium for transmitting the adjusted optical signal, an optical input detection unit for detecting the power of the optical signal from the transmission medium, an optical control-information generating unit for generating information involving the size of the detected optical power monitor value, an optical control-information input introduction unit for introducing the generated information to the transmission medium as optical control information, an optical control-information input detection unit for detecting optical control information from the transmission medium, and a control unit for controlling the adjusted optical power of the optical power control unit within a specified value by means of the detected information.